The invention relates generally to polymeric positive temperature coefficient (PTC) compositions and electrical PTC devices. In particular, the invention relates to polymeric PTC electrical devices that have a high voltage capability and are capable of operating, for example, at alternating current (AC) voltages of 110 to 130 volts or greater, such as those present in household AC electrical lines. Such devices are useful as self-resettable sensors to protect AC motors from damage, such as that caused by over-temperature or over-current surge.
Electrical devices comprising conductive polymeric compositions that exhibit a PTC effect are well known in electronic industries and have many applications, including their use as constant temperature heaters, thermal sensors, over current regulators and low-power circuit protectors. A typical conductive polymeric PTC composition comprises a matrix of a crystalline or semi-crystalline thermoplastic resin (e.g., polyethylene) or an amorphous thermoset resin (e.g., epoxy resin) containing a dispersion of a conductive filler, such as carbon black, graphite chopped fibers, nickel particles or silver flakes. Some compositions additionally contain non-conductive fillers, such as metal oxides, flame retardants, stabilizers, antioxidants, antiozonants, crosslinking agents and dispersing agents.
At a low temperature (e.g. room temperature), the polymeric PTC composition has a contiguous structure that provides a conducting path for an electrical current, presenting low resistivity. However, when a PTC device comprising the composition is heated or an over current causes the device to self-heat to a transition temperature, a less ordered polymer structure resulting from a large thermal expansion presents a high resistivity. In electrical PTC devices, for example, this high resistivity limits the load current, leading to circuit shut off. In the context of this invention, T.sub.S is used to denote the "switching" temperature at which the "PTC effect" (a rapid increase in resistivity) takes place. The sharpness of the resistivity change as plotted on a resistance versus temperature curve is denoted as "squareness", i e., the more vertical the curve at the T.sub.S, the smaller is the temperature range over which the resistivity changes from the low to the maximum values. When the device is cooled to the low temperature value, the resistivity will theoretically return to its previous value. However, in practice, the low-temperature resistivity of the polymeric PTC composition may progressively increase as the number of low-high-low temperature cycles increases, an electrical instability effect known as "ratcheting". Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inorganic fillers or organic additives are usually employed to improve electrical stability.
In the preparation of the conductive PTC polymeric compositions, the processing temperature often exceeds the melting point of the polymer by 20.degree. C. or more, with the result that the polymers may undergo some decomposition or oxidation during the forming process. In addition, some devices exhibit thermal instability at high temperatures and/or high voltages that may result in aging of the polymer. Thus, inorganic fillers and/or antioxidants, etc. may be employed to provide thermal stability.
Polymeric PTC materials have found a variety of applications, such as self-regulating heaters and self-resettable sensors to protect equipment from damage caused by over-temperature or over-current surge. For circuit protection, the polymeric PTC devices are normally required to have the ability to self-reset, to have a low resistivity at 25.degree. C. (10 .OMEGA.cm or less), and to have a moderately high PTC effect (10.sup.3 or higher) in order to withstand a direct current (DC) voltage of 16 to 20 volts for most applications. Polyolefins, particularly polyethylene (PE)-based conductive materials, have been widely explored and employed in these low DC voltage applications. However, there are no currently available polymeric PTC sensor devices that are capable of operating at much higher voltages, such as the 110 to 130 alternating current voltages (VAC) ("Line" voltages) present in AC electrical lines, in which the effective AC current may have peaks equivalent to 156 to 184 DC volts. Polymeric PTC devices able to withstand 110 to 130 VAC would be extremely useful as self-resettable sensors to protect AC motors from damage caused by over-temperature or over-current surge. In particular, such high voltage capacity polymeric PTC devices would be useful to protect the motors of household appliances, such as dishwashers, washers, refrigerators, and the like.
A variety of conductive materials with a polymer matrix including polypropylene, polyvinyl chloride, polyvinylidene fluoride, polyamides (nylon-6, nylon-6,6, nylon-6,10 and nylon-11), polyesters and epoxy have been investigated in DC voltage applications, but most of these polymers did not exhibit both a sufficiently high PTC effect with the required low resistivity to serve as very high voltage protection devices. For example, a conductive nylon-11 composition cited in U.S. Pat. No. 4,304,987 showed a change in resistance at its switching temperature (T.sub.S) of less than one order of magnitude. A conductive PTC composition comprising a polymer blend of high density PE/copolymer of ethylene and butyl acrylate (EBA), disclosed in U.S. Pat. Nos. 5,580,493 and 5,582,770, exhibits low resistivity (1 .OMEGA.cm) and DC voltage capability up to 100 volts; but this is still short of the required capability for withstanding a typical Line voltage of 110 to 130 VAC.
We have recently disclosed high temperature polymeric PTC compositions and PTC electrical devices comprising nylon-12 and/or nylon-11 based conductive materials, in U.S. patent application Ser. Nos. 08/729,822 and 09/046,853, filed Oct. 8, 1996 and Mar. 24, 1998, respectively. The disclosures of each of these patent applications are hereby incorporated by reference in their entirety. These nylon-12 and nylon-11 polymeric PTC compositions have very high switching temperatures (T.sub.s) of greater than 125.degree. C. or greater than 150.degree. C., respectively, and a resistivity at the T.sub.s that is at least 10.sup.3 times, and usually greater than 10.sup.4 times, the resistivity at 25.degree. C. Many of the compositions showed switching temperatures of greater than 160.degree. C. Moreover, some of the compositions and devices fabricated from these compositions had a low resistivity of about 1 .OMEGA.cm and high voltage capability of about 100 DC volts for a single cycle without failure.
In view of the foregoing, there is a need for the development of polymeric PTC compositions, and PTC devices comprising them, that exhibit a high PTC effect, have a low initial resistivity, that exhibit substantial electrical and thermal stability, and that are capable of withstanding very high voltages, such as 110 to 130 VAC or greater, to protect AC motors used by equipment, such as household appliances and the like.